Evaluation of human blood cells is a valuable medical procedure. For example, detection of morphological changes in certain cells or abnormal counts of certain types of cells can lead to diagnoses of certain diseases. But blood cell evaluation is a labor-intensive procedure that involves various instruments, such as centrifuges, spectrophotometers, counting chambers with etched grids, and stained wedge smears of blood. Also blood cell evaluation requires that various reports on red cell indices, white cell counts, white blood cell differentials, and platelet counts be collected.
Hospitals usually rely on automated instruments to perform testing on blood smears. Most automated hematology instruments employ one of the following two methods to measure blood cells. The first method uses an impedance technique to measure the number of blood cells in a test sample. During a test, as blood cells pass through an aperture in a single file, the momentary reduction in conductivity caused by each passing blood cell is detected and counted. The second method relies on optical means. During a test, hydrodynamic focusing technique is used to force the diluted blood cells in the test sample to pass through a flow cell/chamber one by one. As the cells pass through the flow chamber, light beams are focused on the blood sample. As the light beams are scattered by the blood cells in the sample, both forward and side scattered light is analyzed to study the sizes, numbers, and granularities of both white and red cells in the test sample.
Automated blood cell analysis can be performed with little or no human intervention. And the entire analysis only lasts for several minutes. But, according to Patrick C. J. Ward6, automated hematology is not a perfect technology even 40 years after its invention. Identifications of irregular cells, such as immature red or white blood cells, blast cells, circulating lymphoma cells, or atypical lymphocytes, still require a hematologist. Usually, when detecting an irregular cell, an automated device would flag the test for further review by a hematologist. According to a 2006 study conducted by the College of American Pathologists, of the 263 laboratories surveyed, an average of 29% of the automated CBC (complete blood count) results required a manual review. The percentage of test results requiring a manual review could increase dramatically in certain pathology studies. Further, in cases where abnormal lymphoid populations are present, most automated hematology analyzers cannot distinguish between small lymphoid blasts, circulating small lymphoma cells, and normal lymphocytes, and often fail to flag these conditions for further diagnosis.
In today's blood tests, examinations by technicians to identify white blood cell differentials and morphology of white blood cells, red blood cells and platelets are essential. During a manual slide review, a technician well trained in morphologic hematology can distinguish abnormal red cell shapes induced during a staining process from pathologically abnormal red cells. Other challenging tasks that call for a manual blood slide review include identifying fragmented cell forms indicating angiopathic hemolytic anemia (e.g., those associated with disseminated intravascular coagulation) and detecting the presence of immature red and white blood cells and blasts, an indicator of myelodysplastic syndromes or acute leukemia.
Further a manual slide review requires blood films to be prepared in advance, which is a challenging task as well. Wedge blood smear, cover glass smear and spun blood smear methods are three methods commonly used to prepare blood films. The wedge blood smear method can be used in both manual and automated tests. In the wedge blood smear method, a drop of blood is placed about 0.5 inch from one end of a first slide. A second spreader slide is then used to spread the blood into a thin film. The second spreader slide is at about 30°-45° angle relative to the first slide. The blood film generated by the spreader slide sliding along the first slide has a variable thickness. Generally, the part of the film farthest away from the starting point will be too thin, which may cause morphological alterations. The part of the film close to the start of the push will be too thick for microscopy examination. A technician or an automatic slide inspector must find the optimal area for inspection. Also in a wedge blood smear test, the quality of the blood smear depends on a number of factors, for example, the technique used, the viscosity of the blood, the blood smear drying process, the staining process and the environment (such as temperature and humidity). Reference U.S. Pat. No. 5,779,982 describes an automatic blood smear preparing apparatus.
The second method is the cover glass method. In a cover glass method, a small drop of blood is first spread using capillary action generated between two cover glasses. The cover glasses are then pulled apart smoothly in a horizontal plane. However, the cover glass slides used in a cover glass method are often small and difficult to label. In addition, the cover glass method involves higher biohazard risk than the wedge blood smear method. The cover glass method is generally not recommended and has become almost obsolete.
The third method is the spun blood film method. The spun blood film method has been described as an alternative to the wedge blood smear method. It can be automated just as the wedge blood smear method. In a spun blood film method, the blood cells in a test sample are spread over a glass using centrifugal forces. The blood cells form a monolayer that can be stained before microscopic examination. When properly prepared, the morphological condition of all cell types in a spun blood film is generally excellent, although care has to be taken to avoid the formation of smudge cells. However, the instruments used to generate spun blood films, the so-called “cell spinners”, are hazardous because of the formation of droplets and blood aerosols. Also the interior of a cell spinner is almost always contaminated with blood from previous tests, which may cause cross contaminations among test sample and is a potential source of biohazard. Prior art U.S. Pat. Nos. 4,305,722 and 4,016,828 describe a method of preparing blood films on a microscope slide using the spun blood film method.
Conventionally, an automatic blood smear preparation method uses microscope slides as the substrate for carrying the blood film. Microscope slides are a convenient media for manual examination because they have already been used with manual microscopes for other purposes and they can be stored for later review. However, microscope slides take up valuable lab storage space and it is often a tedious task to retrieve a stored slide for review. As such, conventional microscope slides often seem inefficient and outdated.
In hematology analyzers that are currently available, up to 30% of blood test samples are flagged by automatic hematology analyzers for need of human review. Also processes of smear making, smear staining, and blood cell image recording are carried out separately on conventional microscope slides. There is a need for improvements on current hematology analyzers.